Two essential techniques have been employed for the polymerization of fluoromonomers. The most inexpensive and commercially employed technique involves the use of a water-soluble polymerization initiator, and is known as an "aqueous polymerization system." The disadvantage of aqueous polymerization systems is that the resulting fluoropolymers have unstable end groups which are undesirable for many applications. Some of these end groups, such as ionic end groups, carboxylic acid end groups and acid fluoride end groups are hydrolytically unstable or decompose under specific processing techniques, resulting in bubbles or discolorations. These undesirable end groups can be avoided through the use of nonaqueous polymerization systems.
One needs to be careful when choosing a non aqueous medium for fluoromonomer polymerization systems so that the medium does not lead to excessive chain transfer reactions. This is because upon treatment of a fluoromonomer, such as tetrafluoroethylene, with a free radical initiator, the result is a propagating radical chain end that is highly electrophilic. Such highly electrophilic fluorinated radicals are prone to extensive chain transfer to conventional solvents. In addition, in nonaqueous polymerization systems, the initiator must be relatively soluble in the nonaqueous medium employed and the initiator must be chosen to give polymers that have stable end groups which do not decompose during processing. One nonaqueous polymerization system involves the use of fluorinated initiators and chloroflurocarbon polymerization media, such as that described in U.S. Pat. No. 3,642,742 to Carlson. However, in light of the identification of chlorofluorocarbons as one of the primary factors in atmospheric ozone depletion, extensive effort has been directed toward discovering alternative polymerization media systems for fluoropolymerizations.
U.S. Pat. No. 3,642,742 to Carlson discloses one alternative polymerization system involving the use of a fluorinated initiator, a hydrogen containing chain transfer agent, and a fluorocarbon medium. U.S. Pat. No. 5,182,342 to Feiring et al. discloses the use of hydrofluorocarbons as yet another alternative to chlorofluorocarbon systems. Perfluorocarbon and hydrofluorocarbon media are disadvantageous in that they are expensive. Other fluoropolymerization media which have been explored include perfluoroalkyl sulfide fluids, as described in U.S. Pat. No. 5,286,822 to Krespam et al., and perfluorinated cyclic amines, as described in U.S. Pat. No. 5,310,836 to Treat. However, these media are also expensive.
As an alternative polymerization media, researchers have recently begun exploring the use of carbon dioxide as a polymerization medium. For example, U.S. Pat. No. 3,522,228 to Fukui et al. discloses the polymerization of vinyl monomers using peroxide polymerization initiators in liquid carbon dioxide at temperatures from -78.degree. C. to 100.degree. C. Fukui provides no example of a fluoromonomer polymerization in a carbon dioxide solvent, and indeed the methods disclosed therein have failed to achieve commercial utility for the preparation of fluoropolymers. In addition, Fukui fails to recognize the important role of stable end groups in fluoropolymers. U.S. Pat. No. 5,328,972 to Dada et al. discloses a process for the production of low molecular weight polymers of C.sub.3 -C.sub.4 monoethyleneically unsaturated monocarboxylic acids in supercritical carbon dioxide at temperatures of at least 200.degree. C. and pressures above 3,500 psi. U.S. Pat. No. 5,345,013 to Van Bramer et al. proposes mixtures of tetrafluoroethylene monomer and carbon dioxide for safe handling, but does not discuss fluoropolymerization methods.
PCT Publication No. WO 93/20116 to University of North Carolina at Chapel Hill discloses processes for making fluoropolymers comprising solubilizing a fluoromonomer in a solvent comprising carbon dioxide. PCT Publication No. 93 WO/20116 is not concerned with the problem of polymerizing fluoromonomers to provide polymers having stable end groups.
Accordingly, there remains a need in the art for a method of making fluoropolymers having stable end groups which avoids the use of polymerization media which are detrimental to the environment, such as chlorofluorocarbons. There is also a need in the art for fluoropolymerization processes capable of commercialization, which produce fluoropolymers having stable end groups, and which utilize nonhazardous, relatively inexpensive polymerization media which are relatively easily separable from the fluoropolymer produced.